1. Field of the Invention
The present invention relates to a vertical velocity modulation apparatus having the function of correcting the image quality by modulating the scanning speed of an electron beam, and a video display apparatus and a video display method using the same.
2. Description of the Related Art
In a conventional video display apparatus such as a television receiver or a display monitor, a unidirectional scanning system has been generally used. FIG. 31 is a diagram showing a conventional unidirectional progressive scanning system. As shown in FIG. 31, in the unidirectional progressive scanning system, scanning is performed slightly downward from the left to the right of a screen. In a progressive scanning system of 525 scanning lines, the horizontal scanning frequency is 31.5 KHz, and the horizontal scanning period is 31.75 μs.
In recent years, a bidirectional scanning system has been proposed in order to improve the image quality. FIG. 32 is a diagram showing a bidirectional progressive scanning system. As shown in FIG. 32, scanning lines in odd numbers are horizontally scanned from the left to the right of a screen, and scanning lines in even numbers are horizontally scanned from the right to the left of the screen. In a bidirectional progressive scanning system of 1050 scanning lines, the horizontal scanning frequency is 63.0 KHz, and the horizontal scanning period is 15.87 μs.
In the bidirectional scanning system, the density of the scanning lines in the vertical direction doubles, so that the resolution is increased, and the luminance is improved. Since reciprocating scanning is performed, power for deflecting an electron beam is reduced, thereby making it possible to reduce the scale of a power supply circuit.
In the bidirectional scanning system, however, the interval between the scanning lines is shortened, so that the luminance of each of the scanning lines is easily affected by the luminance of the adjacent scanning line. Consequently, the sharpness of a reproduced image may, in some cases, be reduced.
For example, Japanese Laid-open Patent 10-23290 discloses that the velocity of an electron beam is modulated in the vertical direction, to make vertical contour correction.
In velocity modulation in the vertical direction which is disclosed in Japanese Laid-open Patent 10-23290 , a scanning line having a low luminance is brought near a scanning line having a high luminance at a change point from a low luminance level to a high luminance level, and a scanning line having a high luminance is brought near a scanning line having a low luminance at a change point from a high luminance level to a low luminance level. For example, a scanning line h2 having a low luminance is shifted by Δp downward, and a scanning line h5 having a high luminance is shifted by Δp upward.
As described in Japanese Laid-open Patent 10-23290 , the luminance is high in a portion where the interval between the scanning lines is narrow, while being low in a portion where the interval between the scanning lines is wide. As a result, the change in the luminance of the luminance change portion, for example, a vertical contour portion of an image can be sharp.
According to the conventional method of modulating the scanning speed in the vertical direction, however, there arises such a phenomenon that an originally dark portion is brightened, so that the contour is rather blurred.
An object of the present invention is to provide a vertical velocity modulation apparatus capable of obtaining a sharper reproduced image by modulating the scanning speed in the vertical direction, and a video display apparatus and a video display method using the same.
Another object of the present invention is to provide a vertical velocity modulation apparatus which can obtain a sharper reproduced image by modulating the scanning speed in the vertical direction and can be miniaturized and reduced in cost, and a video display apparatus and a video display method using the same.